hello!i tried to install ubuntu from cd on my external usb hard disk.i installed it from windows as it was a program.i chose the correct device and i give it 30Gb.it install and in the end told me to reboot.i did it and i chose ubuntu from the grub menu.it told me it hasn't finished and after a few minute loading with ubuntu logo it gave me an error something like "buffering I/O error" on a certain block.it tried twice more and nothing happend.can you help me plese?thank you!!!